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II                           MAMMALS   OF   ARCTIC REGION                        81
that "but little reliance can be placed on them as a means of
transit; besides, here again, two individuals, or a pregnant female,
would be required to effect a settlement on a foreign shore.
The existence of oceanic islands is often urged as a proof of this
inability to cross tracts of sea; even those which are com-
paratively near an extensive continent, such as, for example,
Fernando IsToronha in the Atlantic, are destitute of mammals
(except, indeed, the ubiquitous Mouse, which is believed to have
been carried there, often in company with the equally widely-
spread Hat, in ships). This argument, however, is not so conclusive
as might appear ; it doubtless is in the case of far-distant islands.
But the size of the islands has to be taken into account. ITor
there are islands, such as the Galapagos, or, to take a less con-
tested instance, some of the islands of the Malagasy Archipelago,
undoubtedly continental, which have an exceedingly reduced
number of mammals. An area of a certain size seems to be a
The converse of this is in many cases easy to show, that is,
the wide range of animals when there are no marine barriers to
stop their spreading. John Hunter, the celebrated anatomist
and surgeon (not often quoted, however, as an authority upon
geographical distribution), observes: " It is a curious circum-
stance in the natural history of animals to find most of the
northern animals the same both on the continent of America and
what is called the Old World, while those of the warmer parts
of both continents are not so. Thus we find the bear, fox, wolf,
elk, reindeer, ptarmigan, etc., in the northern parts of both. . . .
The reason why the same animals are to be found in the northern
parts is the nearness of the two continents. They are so near
as to be within the power of accident to bring the animals,
especially the large ones, from one continent to the other either
on the ice or even by water. But the continents diverging from
each other southward, so as to be at a very considerable distance
from each other even beyond the flight of birds, is the reason
why the quadrupeds are not the same."
There is no doubt, in fact, that the ocean is the most in-
superable of all barriers to the dispersal of mammals. In a less
degree mountain ranges and deserts are also barriera The
Desert of Sahara is a striking instance to the point; it separates
two exceedingly different faunas^
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